1. Field of the Invention
This invention relates to a preferred image processing system and a preferred gamut adjustment method used for gamut adjustments of copiers, printers, and image output devices.
2. Description of the Related Art
Image output devices of displays, printers, etc., include a reproducible color range (gamut). Generally, the gamut of a printer is narrower than that of a display. Therefore, if the user makes color adjustments while seeing an image displayed on a display and obtains a satisfactory image, when the image data is output to a printer as it is, the intended output image may not be provided.
To output an image by using a device with a narrow gamut, generally image data is previously compressed within the gamut of the output device (gamut compression). Various gamut compression systems are known. The main three systems will be discussed with reference to FIGS. 10A to 10C.
(a) Common area storage type (calorimetric match)
FIGS. 10A shows L*a*b* space, wherein narrow area GOUT surrounding origin O. The area surrounding the output gamut GOUT is the gamut of original image data (before gamut adjustment). In the common area storage type, colors in the common area to input and output gamuts GIN and GOUT are stored as they are. In the example shown in the figure, all area of the output gamut GOUT is contained in the input gamut GIN, thus if colors in input image data are contained in the output gamut GOUT, a color adjustment is not made to the colors.
On the other hand, if colors in input image data are outside the range of the output gamut GOUT, they are moved as indicated by arrows in the figure. That is, the colors are moved toward a direction orthogonal to a lightness axis (L* axis) and are replaced with outermost colors in the output gamut GOUT (boundary colors).
Since the lightness is stored, the system is appropriate for applications where accuracy of lightness is required, such as reproduction of corporate color. On the other hand, the system has disadvantages in that chromatic colors are converted into achromatic colors in areas high and low in lightness. It also has disadvantages in that gradation is lost and that colors are crushed because all colors outside the gamut are converted into boundary colors.
(b) Saturation storage type (saturation match)
In the system shown in FIG. 10B, colors in the common area to input and output gamuts GIN and GOUT are stored as they are, as in FIG. 10A. On the other hand, if colors in input image data are outside the range of the output gamut GOUT, they are moved as indicated by arrows in FIG. 10B. That is, the colors are moved toward the origin of the L*a*b* space and are replaced with outermost colors in the output gamut GOUT (boundary colors).
This system can store saturation as much as possible without changing any hue. Therefore, it is appropriate for enhancing color differences and easily distinguishing colors in business graphs, computer graphics, etc. However, the system has disadvantages in that gradation characteristics become incorrect because lightness is changed. Like the system in FIG. 10A, the system in FIG. 10B also has disadvantages in that colors are crushed.
(c) Relative lightness storage type (perceptual match)
In the system shown in FIG. 10C, colors are moved toward origin O, as in the system in FIG. 10B. However, all colors in color space are moved. Here, imagining a straight line toward the color to be converted from the origin O and assuming that the length from the origin O to the boundary color of output gamut GOUT along the straight line is x1 and that the distance to the boundary color of input gamut GIN is x2, each color is adjusted so that the distance to the origin O becomes x1/x2 times.
The system, which can hold gradation characteristics over the whole of image data after gamut adjustment, can prevent color crushing, etc., and is appropriate for reproduction of natural images. However, since colors in the output gamut GOUT are also moved toward the origin O, the system has disadvantages in that it is poor in saturation of reproduction colors and cannot make the effective use of the output gamut GOUT.
As we have discussed, each of the gamut compression systems has its merits and demerits, and it is difficult to general users to decide which system is suitable for specific original images. All the systems discussed above may also be improper. In such a case, a more appropriate image may be able to be provided by mixing the results of the color adjustments.